I've always been big on "sexual education" in the get the job done sense and was fortunate to have a boyfriend who shared the academic interest.
We mutually delved into a lot of guides in different formats that had material ranging from kamasutra, foreplay, positions, tips & techniques, and so on. If you already think yourself fairly well versed, most of what you find or purchase is going to be directed at men, and will only contain a few good tidbits or facts here and there.
The material I found useful was in the form of novel bedroom techniques and "why didn't I think of that!" stuff. For example, the use of a fine soft scarf to rub across each other as foreplay, wines and foods that best mix with the flavors of sex, and even placing things between your legs when being penetrated from behind so your clitoris has something to gently brush against. I'll see if I can find what the book that touched on this stuff was... I think it was a 100-best or something stupid like that, but was divided half-and-half for men and women and was well written.
Nina Hartley has some great guides for both sexes, and unique ideas to boot! Things that sound silly at first like tracing the motions on the underside of his chin with your finger, so he knows exactly how you want him to move whatever part of his body actually works *reallly* well. Thank-you Nina!
She Comes First: The Thinking Man's Guide To Pleasuring A Woman: Amazon.ca: Ian Kerner: Books
Was the book *I* appreciated most, and was the biggest recipient of. If you're finding the foreplay of your partner to be in need of improvement (or you want to glean some ideas for yourself), this would be my first go-to.
Also, I am certain I read the Lesbian Sex Book mentioned above (though it had a different cover) and give it another nod of approval.